The design of proteins with improved or novel functions is an important goal with a variety of medical, industrial, environmental, and basic research applications. Now that combinatorial antibody libraries have been secured,, a powerful next step is the evolution toward artificial constructs as well as other protein motifs in which dimeric species are native or might be functional. Recently, we developed a phage-display format for the construction of highly diverse heterodimeric polypeptide arrays. With this approach, pVIII and pIX are utilized for the display of fusion proteins that form dimeric species. It is important to note that this is an entirely new methodology because one independently display protein motifs in close proximity to generate a library of functional interactions. The main goal of this proposal is to explore the potential of this approach for protein design. Inherent in the scope and power of the technology is the ability to display a variety of proteins that can engage in dimeric interactions. These include not only antibodies, but also some enzymes, hormones and hormone receptors, and DNA binding proteins. First, experiments have been designed to incorporate either combinatorial alteration of antibody framework regions and in other studies to reorganize and miniaturize the antibody chain for the creation of dimeric artificial amidases. Finally, we will display the phage 434 repressor protein as a library of heterodimers for selection against particular DNA sequences of clinical therapeutic importance. Our investigations represent a first step toward artificial antibodies and other protein motifs that will increase the understanding of protein- protein interactions and facilitate the selection of novel biological activities.